This is a departure from the Poirot norm for Christie, which I don't think really worked. Rather than a whodunit, it's more of a thriller akin to James Bond (though they came later) complete with megalomanic baddies and a secret mountain hideaway.
Poirot is on the trail of 'The Big Four' a master criminal gang intent on taking over the world. Through a number of individual encounters Poirot tracks the four down and plots their downfall, assisted by the inept narrator, Captain Hastings.
The plot is structured almost as a series of short-stories, each building slightly upon the last and gradually leading towards the conclussion. As such it doesn't really fit together as a novel, and unlike the usual Poirot books there is no mystery for the reader to attempt to solve along with Poirot - we are just left to follow Hastings as he bumbles along, oblivious to Poirot's plans. This makes it quite a frustrating read as sometimes you are deliberately left in the dark, and at others it's so obvious what's going on that you want to kick Hastings for not falling in.
So overall it's only a mildly entertaining read, and certainly not one that demands a great deal of brain power. I hope this was a one off and we get back to the Christie cliché of 'murder at the manor'.